In today's paperless office, electronic messages are instantaneously distributed throughout the office computer system. Each message originates from the memory of a transmitting station and is received into the memory of one or more receiving stations. The basic "party" network permits only one station to send at a time, creating a potential for interruptions of an existing transmission, and the collision of two transmissions starting at the same time.
In the prior art "post-transmission" technique, each message is automatically resent to the source to be verified against the original message. Errors are not detected until the message has been transmitted twice. Erroneous messages must be resent and reverified until completed without interruption or collision. The costly re-transmission penalty limits the post-transmission approach to small, low volume communication systems.
Interruption prevention circuitry is required to latch the remaining, non-transmitting stations, into the receiving mode during in-coming transmissions. Communication collisions between two stations which simultaneously acquire transmission status is a less frequent (a special case of double transmission not covered by interrupt prevention). Simultaneous starts must be identified and aborted within an initial transmission test period. In addition, post collision priorities must be established to resolve the simultaneous start.
System wide controls in prior art networks provide for organized use of the party network, and double transmissions are avoided. Each subsequent user is identified in advance through user ques and transmission rosters. The system controls require more overhead time, additional software, and circuitry. Interrupts and collisions are avoided at the expense of speed and simplicity.
Prior art, single conductor coaxial cables have been dc coupled between member stations to indicate the receive-transmit status of the user stations. The coaxial shield was maintained at circuit ground, and therefore failed to isolate the stations from static discharge accumulating along the extensive party cable and from magnetic start-up surges associated with motors, flourescent lamps, and capacitive type power supplies.